Ultrasonic diagnostic apparatuses utilizing a pulse-echo method that transmit pulsed ultrasonic waves to a living body and receive the reflected waves thus imaging the inside of the living body are widely used for medical diagnosis, as well as X-ray and MRI.
In order to achieve three-dimensional imaging for medical diagnosis with use of a two-dimensional array of ultrasound transducers, the number of signal lines led out from the transducers poses a problem. That is, because the two-dimensional array needs about 103 to 104 transducers in total, if a signal line is individually led out from every transducer, the number of the signal lines will be so great that the connection cable becomes too thick to handle.
In order to solve this problem, a method is disclosed in Japanese Patent Application Laid-Open Publication No. 2001-286467 (hereinafter called a reference 1), where switch circuitry is mounted on a two-dimensional array of ultrasound transducers, and elements forming the array are connected together as needed via the switch circuitry connecting to a cable, thereby reducing the number of cable cores led out by the order of one or two digits. The phase distribution at the plane reception surface of ultrasonic waves emitted from the focal position takes the form of concentric circles. Hence, in the reference 1, elements on the same circle of the concentric circles are connected together to the same cable core so as to lead out a signal. Further, because the pattern for connecting elements together needs to vary according to the beam formation direction, the connection pattern is changed using the switch circuitry.
In achieving three-dimensional imaging with use of a two-dimensional array of ultrasound transducers, another problem is with forming a plurality of beams simultaneously. High information-acquisition throughput is needed to acquire a large amount of image information necessary to form a three-dimensional image with utilizing high time resolution characteristic of the ultrasound imaging. Thus, the use of multiple beams for the simultaneous transmit/receive beam is indispensable. However, when using the element connection patterns of the reference 1 as they are, only one transmit/receive beam is formed corresponding to one pattern. Thus, this method is not suitable for high speed imaging.
Accordingly, a first object of the present invention is to provide an ultrasound imaging apparatus capable of simultaneously forming multiple beams suitable for high speed imaging at a low cost.
In order to realize the dynamic state of a dynamic part of an object three-dimensionally in real time by, for example, the observation of blood flow through a coronary artery of the heart and the measurement of systolic output, it was considered to obtain three-dimensional images in real time using an ultrasound probe comprising a two-dimensional array having electro-acoustic transducer elements arranged in a plane. However, because there was a conflicting relationship between the breadth of the field of view (the depth and viewing angle), the height of resolution, and the height of a frame rate (real-time capability), in order to improve an element, another element had to be sacrificed.
For example, assuming that the viewing angle is 60 degrees in both the lateral axis direction and elevational axis direction of the two-dimensional array and that the scan line interval is 1.5 degrees, then the number of scan lines per frame is 1,600. In order to obtain images of an object up to a depth of, e.g., 20 cm (the both-way distance for ultrasonic waves being 40 cm), scan time per scan line is at least about 260 μs because the speed of sound in usual parts of a living body is about 1,530 m/s. Thus, in this case, the frame cycle is about 0.4 sec and the frame rate is about 2.5 Hz, so that a frame rate of 20 to 30 Hz or greater, which is necessary for the observation of the cardiac dynamic state, could not be achieved.
Accordingly, in, e.g., Japanese Patent No. 2961903 (paragraphs 0008-0009, FIG. 3), an ultrasound three-dimensional imaging apparatus has been proposed which has a phase adjusting circuit that adjusts the phases of received signals output from a two-dimensional array of oscillators, which are divided into groups, to simultaneously form, e.g., four receive beams deflected at different small angles relative to a transmit direction.
Moreover, in Japanese Patent Application Laid-Open Publication No. 2000-33087 (paragraph 0090, FIG. 11), a phased array acoustic apparatus with in-group processors has been proposed where an array of 3,000 transducers is divided into 120 groups, or sub-arrays, each comprising 25 transducers and where an in-group processor delays and sums individual transducer signals and supplies the summed signal to one channel of a receive beam former.
Furthermore, in Japanese Patent Application Laid-Open Publication No. 2001-286467 (paragraph 0021, FIG. 3), an ultrasound diagnostic apparatus has been proposed where a delay corresponding to the distance to the focal point is given to each group of oscillators in a concentric annular area of a two-dimensional oscillator array such that ultrasonic waves emitted from each ring-like group of oscillators are converged on the focal point and that the ultrasonic waves reflected from the focal point are directed to the ring-like group of oscillators.
With a conventional ultrasound three-dimensional imaging apparatus, if four receive beams are formed simultaneously for one transmit beam, with the same breadth of the area to be imaged and the same resolution, a frame rate will quadruple. Hence, in the above example, in order to achieve a frame rate of 20 Hz or greater, eight or more ultrasound receive beams need to be formed for one ultrasound transmit beam.
However, in order to obtain images of sufficient resolution, a two-dimensional array of several thousand oscillators needs to be used. Accordingly, the conventional ultrasound imaging apparatus requires several thousand delay means and summing means, so that the size of a delay-and-sum circuit becomes huge. Thus, there is the problem that it is difficult to realize the apparatus as well as production costs being high. Further, if it is produced, the number of connection lines from the two-dimensional array of oscillators will be several thousand, resulting in imaging operation being actually impossible.
Generally, in order to obtain sufficient resolution, the aperture length of the two-dimensional array of oscillators needs to be made as large as possible to use a large number of electro-acoustic transducer elements. However, a receive beam former having several thousand input channels is unrealistic in terms of circuit size. Hence, it has been considered to reduce several thousand channels of electro-acoustic transducer elements to about 100 to 200 channels.
With the conventional phased array acoustic apparatus with in-group processors, because the number of channels is reduced, the circuit size is reduced and thus the improvement in operability can be expected. However, a grating lobe may occur depending on the shape of the sub-arrays of transducer elements. Thus, sufficient resolution and contrast may not be obtained, or noise or a false image may occur, so that desired image quality may not be obtained. Further, if more finely grouped, the number of channels increases and the circuit size increases, so that a desired frame rate may not be achieved.
Moreover, with the conventional ultrasound diagnostic apparatus, there are the following problems. Because the ring width of each ring-like group of oscillators is constant (a pitch of two elements), the intervals between the groups may be almost equal to the wavelength (the pitch of two elements) depending on the direction in which the ultrasound beam is directed, and thus a large grating lobe may occur and degrade image quality. If the intervals between the groups are decreased to suppress the occurrence of a grating lobe, the circuit size increases. Further, because the number of oscillators is extremely different between the inner ring and the outer ring, electrical characteristics such as impedance are greatly different for each ring. Thus, the size of circuitry for correction becomes large, or image quality is reduced. Or, if thinning the oscillators out so as to make the electrical characteristics the same for each ring, resolution will be reduced.
As such, in the case of reducing the number of channels of electro-acoustic transducer elements by grouping them, there is the problem that, because a conflicting relationship exists between reducing the number of channels and suppressing a grating lobe, as the number of channels is reduced, image quality is degraded.
Accordingly, a second object of the present invention is to solve the above problems and provide an ultrasound imaging apparatus that can produce ultrasound three-dimensional images with a broad field of view, high resolution, and a high frame rate at low cost.